The United States and Japan will step up their defence cooperation to deal with the threat from nuclear-armed North Korea as tensions in East Asia remain high, officials from the two allies said on Thursday.

They are also charged with conspiring to endanger the life of Detective Inspector Craig Wall.

Opening the Crown case in the South Australian Supreme Court on Wednesday, prosecutor Sandi McDonald said Walkuski was seeking revenge against Mr Rice, who had sentenced him to 17 years in prison, when he organised for Van Shaick to carry out the killings on his release.

Walkuski had expected to get about half that much prison time, Ms McDonald told the Supreme Court.

A third inmate was also invited by Walkuski to be involved in the plot but he declined.